Adsorbents are commonly solid materials which exhibit selectivity at their surface for substances in a mixture, thereby providing a means of separating such substances from the mixture. The high surface area characteristic of adsorbents (usually well above 5 m.sup.2 /g of material) normally results from a fine particle size (non-porous adsorbents) or from pores in the adsorbent particles (porous adsorbents). Carbon black, TiO.sub.2 and ZnO pigments are examples of non-porous adsorbents. Granular carbon, silica gel, bone char, certain soils and asbestos are examples of well-known porous adsorbents obtained from naturally occurring materials. These suffer seriously from high moisture adsorption in humid atmospheres, poor reproducibility during manufacture, and fragmentation during use in cyclic processes, because their rigid structures are broken by the high osmotic pressure of adsorbed materials in their small pores.
For separation or purification of complex substances (such as pharmaceuticals) synthetic adsorbents have been developed, some of which serve also as ion exchange materials or as intermediates for the manufacture of ion exchange materials. However, ion exchange is an absorption as well as an adsorption phenomenon, so that although all ion exchange materials are adsorbents, the converse is not necessarily true.
The synthetic adsorbents generally are porous polymeric solids, polystyrene and styrene-divinylbenzene copolymers being representative. Although it is possible to prepare synthetic polymers that are finely divided or to grind such polymers to a fine particle size, to obtain high surface area, fine particle size adsorbents cannot be used in cyclic processes, particularly processes involving columns, since the fine particles pack too tightly and impede flow. Moderately large adsorbent particles on the order of about 0.02 mm to 2 mm diameter or larger, are therefore required. Polymeric beads, obtained by known suspension polymerization techniques, have a convenient particle size for use in columnar operations. While the polymeric adsorbents can be made hydrophobic and the bead form makes them more useful, their adsorbent properties have been too limited for the adsorbents to compete effectively with the carbonaceous adsorbents obtained by pyrolyzing of organic materials.
Macronet adsorbents in bead form are taught by Reed, U.S. Pat. No. 4,263,407, which is hereby incorporated into the present specification by reference. They are produced by swelling a lightly crosslinked, macroreticular, aromatic polymer bead in an inert organic solvent, and then post-crosslinking the swollen beads with an external crosslinker. These adsorbents are called "macronets" because the crosslinks are stable and have a long and rigid structure which allows the polymer to retain the displacement of the chains to significant distances from one another that occurs during solvent swelling, even after the solvent has been removed.
Itagaki et al., U.S. Pat. No. 4,543,365 disclose bead materials similar to those of Reed, but employing more highly crosslinked resins.
East German Offenlegungschrift 229,992 teaches a one-step preparation of high-surface-area, sulfonic acid resin beads from lightly crosslinked, suspension-polymerized polystyrene beads by chloromethylation and sulfonation in the presence of a swelling solvent; the resulting beads appear to be macronet beads.
Fibers have particular advantages over conventional adsorbents like carbon filaments or granular, activated carbons; they may be woven or otherwise processed into cloth or other textile materials for making garments protective against chemicals, filters for air purification systems, and the like. They, and other polymeric adsorbents, are also easier to regenerate and less sensitive to high humidity than activated carbons.
Fibers, by their small diameter, provide relatively high surface areas, and chemically modified, polystyrene-based fibers are known, as for example those of Yoshioka et al., Bull. Chem. Soc. Japan 56, 3726 (1983) or Japanese Kokai 77-120986. These fibers are composites of a vinylaromatic polymer matrix and longitudinal fibrils of alpha-olefin polymer imbedded in the matrix. The references teach treatment of the vinylaromatic surface of the fibers to attach functional groups such as sulfonic acid groups and amine or ammonium groups.
Japanese Kokai 75-145617 discloses treating phenol-formaldehyde polymer fibers with alkylating reagents such as paraformaldehyde in acidic media, a reaction which the above Yoshioka and Kokai 77-120986 use to prepare the surface for functionalization. In Kokai 75-145617 this reaction is followed by a mild pyrolysis in an oxidizing atmosphere at temperatures from 250.degree. to 450.degree. C.; the reference reports that this treatment enhances the surface area of the fibers to at least 10 m.sup.2 /g and as much as 400 m.sup.2 /g.
Another approach to porous fibers is disclosed by Sruta et al., Chemicke Vlakna, 1986, 36, No. 3, pages 175-181. This reference discloses porous polyester fibers formed when calcium carbonate, spun into the fibers as a delustrant, is dissolved with acid.
An object of the present invention is to provide adsorbent fibers with high adsorptive capacity for gases, vapors and the like. A further object of the present invention is to provide high-surface-area fibers bearing chemical functionality suitable for chemical interactions commonly encountered in adsorptive, organic-reactive or ion-exchange phenomena. Another object of the present invention is to provide an adsorptive filter medium which can provide the adsorption rate of a bed of fine, particulate adsorbers while avoiding the problems of high pressure drop, filter clogging and the like. Yet another object is to provide adsorbent microbeads with a high surface area and high capacity for gases, vapors and the like. Other objects of the present invention will be apparent from the specification.